1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a light guide plate, a liquid crystal display device having the same, and a method for displaying pictures using the liquid crystal display device, in which display function is carried out even in a dark place.
2. Description of the Related Art
A liquid crystal display device is one of display devices, which precisely controls a light transmission of a liquid crystal by using electro-optical features of the liquid crystal so as to allow a user to recognize information processed in an information processing unit.
The liquid crystal display devices are generally classified into a reflection type liquid crystal display device and a transmission type liquid crystal display device. The transmission type liquid crystal display device is mainly used for a middle-sized or a large-sized display device and the reflection type liquid crystal display device is mainly used for a small-sized or a middle-sized display device.
Since the reflection type liquid crystal display device displays information by using an external light source, it has a simple structure. In addition, the reflection type liquid crystal display device has low power consumption when displaying information because it can display information with a little power required for controlling a liquid crystal.
Though the reflection type liquid crystal display device has the simple structure and the low power consumption, it does not precisely display information at night or when the quantity of light is insufficient, since the reflection type liquid crystal display device displays information by receiving the light from the exterior thereof.
Such problems can be solved by using the transmission type liquid crystal display device. The transmission type liquid crystal display device generates the light by consuming an electric energy supplied to the transmission type liquid crystal display device. That is, the transmission type liquid crystal display device can create the light required for displaying information by using the electric energy charged therein, so it can freely display information in any place regardless of an external environmental condition.
However, the transmission type liquid crystal display device needs an additional power to generate the light required for displaying information besides the power for controlling the liquid crystal, so the power consumption thereof increases as compared with that of the reflection type liquid crystal display device.
A front illumination type liquid crystal display device solves the problems of the transmission and reflection type liquid crystal display devices and maintains advantages thereof.
The front illumination type liquid crystal display device displays information by using an external light when the external light is sufficient. On the other hand, when the external light is insufficient, the front illumination type liquid crystal display device displays information by using an artificial light, which is generated by consuming the electric energy charged therein. As a result, the front illumination type liquid crystal display device can display information in any place with a reduced power consumption.
FIG. 1 shows a conventional front illumination type liquid crystal display device 10 (hereinafter, simply referred to as “liquid crystal display device”).
Referring to FIG. 1, the conventional liquid crystal display device 10 mainly includes a light source 2, a light guide plate 4, and a liquid crystal display panel assembly 6.
Though there are not illustrated in FIG. 1, the liquid crystal display panel assembly 6 includes a liquid crystal display panel having a TFT substrate, a liquid crystal, and a color filter substrate and a driving module.
In detail, common electrodes, to which the power is applied with a same intensity, and R.G.B pixels are formed on the color filter substrate. A plurality of pixel electrodes each having a micro surface area, signal lines for supplying a power to each pixel electrode with a different intensity, and thin film transistors are formed on the TFT substrate. The liquid crystal is injected between the color filter substrate and the TFT substrate.
A driving module is provided to process data applied from an external information processing unit. The driving module sends the processed data to the signal lines formed on the TFT substrate, so as to display information.
The liquid crystal display panel assembly 6 has a structure adapted for individually controlling the power outputted from each thin film transistor. The liquid crystal display panel assembly 6 can individually control the intensity of power applied to pixel electrodes connected to each thin film transistor. Consequently, the liquid crystal display panel assembly 6 precisely controls the alignment of the liquid crystal by a microscopic area unit based on the difference of the electric field between the pixel electrodes and common electrodes.
Though it is possible to precisely control the alignment of the liquid crystal by the microscopic area unit, a light source as shown in FIG. 1 is required for generating the light, because the liquid crystal display device does not display information without the light.
It is preferable that the light supplied to the liquid crystal display panel assembly 6 does not represent a brightness variation within a predetermined area, just like sunlight. However, it is very difficult to manufacture the light source having the brightness distribution similar to that of the sunlight, so a linear light source or a point light source which has a high brightness and can be simply manufactured is used for the light source.
Though the linear light source or the point light source is easily manufactured and has a higher local brightness, the brightness distribution thereof varies depending on a distance between the light source and a position which the incident light reaches. Therefore, if the light generated from the linear light source or the point light source is directly supplied to the liquid crystal display panel assembly, it is impossible to obtain a desired image due to the brightness variation even when the liquid crystal is precisely controlled.
For this reason, as shown in FIG. 1, the light guide plate 4 is used for obtaining a surface light source effect, which is similar to the sunlight, from the light generated by the linear light source or the point light source.
The light guide plate 4 has a hexagonal plate shape with a thin thickness, which corresponds to the shape of an effective display area of the liquid crystal display device 10.
The light guide plate 4 receives the light having an optical distribution densely focused in an area remarkably narrower than the effective display area, and varies the optical distribution of the light to have a uniform optical distribution over the effective display area. Then, the light guide plate 4 sends the light to the above-mentioned liquid crystal display assembly 6.
FIG. 2 is an enlarged view of “A” portion shown in FIG. 1;
In order to maximize the optical efficiency by reducing the optical loss at the light guide plate 4, a plurality of light reflection patterns 3 in the form of V-shaped grooves are continuously formed on an upper surface of the light guide plate 4.
However, the light reflection patterns 3 formed on the upper surface of the light guide plate 4 to increase the optical efficiency may generate a light interference phenomenon, called “moire phenomenon”, depending on the alignment of the pixel electrodes of the liquid crystal display panel assembly 6, which are aligned in a matrix pattern.
In detail, as shown in FIG. 3, when an orientation of the light reflection patterns 3 matches the aligning direction of the pixel electrodes 6a of the liquid crystal display panel assembly 6, two patterns are overlapped with each other so that the moiré phenomenon occurs.
The moiré phenomenon can be reduced by tilting the extending direction of the light reflection patterns 3 from the aligning direction of the pixel electrodes 6a by an angle of 22.5 degrees as shown in FIG. 4A.
However, if the extending direction of the light reflection patterns 3 is tilted from the aligning direction of the pixel electrodes 6a by an angle of 22.5 degrees as shown in FIG. 4A, the effective display area is divided into a bright area III, a dark area I, and a boundary area III, that is, a brightness unbalance phenomenon is created as shown in FIG. 4B.
Referring to FIGS. 1 to 4B, the moving direction of the light generated from the lamp 2 in the form of the linear light source varies depending on the reflection angle of the light with respect to the light reflection patterns 3, so the effective display area can be divided into three areas.
That is, when the light generated from the lamp 2 is reflected from the light reflection patterns 3 and is directed towards the pixel electrodes 6a of the liquid crystal display panel assembly 6, the light reaches the bright area III as shown in FIG. 4B. In this area, the displaying function is carried out with a high brightness.
On the contrary, the light generated from the lamp 2 reaches the dark area I shown in FIG. 4B, when the light is not reflected towards the liquid crystal display panel assembly 6, but reflected towards a side of the light guide plate 4 shown in FIG. 4A and leaked out of the exterior. Therefore, the quantity of light at the pixel electrodes 6a of the liquid crystal display panel assembly 6 is insufficient in the dark area I, so information is displayed in the dark area I with a lowered brightness.
On the other hand, the boundary area II is brighter than the dark area I and darker than the bright area III and has a strip shape with a predetermined width. In the boundary area II, the brightness gradually increases as approaching to the bright area III from the dark area I.
Where the effective display area is divided into three areas, though the moiré phenomenon can be prevented, the brightness non-uniformity is remarkably created in each area due to the brightness variation thereof.
Consequently, where the extending direction of the light reflection pattern 3 formed on the light guide plate 4 matches the aligning direction of the pixel electrodes 6a, the moiré phenomenon is created though the effective display area is not divided into the dark area and bright area. On the contrary, where the extending direction of the light reflection pattern 3 formed on the light guide plate 4 is tilted from the aligning direction of the pixel electrodes 6a, the brightness non-uniformity phenomenon is created in the effective display area, though the moiré phenomenon can be reduced.